Abstract: This thesis deals with two important aspects that affect nutrition of the boreal forest: stress and disturbance. Stress in this context refers to the nutritional stress of plants defined as any amount of nutrient that decreases productivity. Disturbance refers to as any event that results in a significant loss of biomass in an ecosystem, thereby altering its structure. These interferences can be either detrimental or beneficial on tree growth, depending on forestry management. In my first chapter, I examined roadside car emissions and their effects on tree nutrition and biological nitrogen fixation by moss-associated cyanobacteria. Experimental protocols included acetylene reduction assays to estimate biological nitrogen fixation of mosses (Pleurozium schreberi) and elemental analyses to measure nutrient and heavy metal concentrations in Norway spruce needles (Picea abies). We also investigated the effect of molybdenum and phosphorus additions on biological nitrogen fixation along a supposed gradient of atmospheric nitrogen deposition produced by exposure to roadside emissions. We observed a decrease in biological nitrogen fixation rates with increasing distance from the roadside. The effect of molybdenum and phosphorus on the biological nitrogen fixation was rather rare and showed no spatial patterns. We did not detect a nitrogen deposition gradient, but we found an increase in heavy metals and phosphorus in leaf tissue near roads, which may explain the decrease in nitrogen fixation rates. My second chapter examines the effects of soil scarification after a clear-cut on the on the nutritional quality of regenerating black spruce trees (Picea mariana). More specifically, we checked whether scarification could reduce nitrogen and phosphorus deficiencies. Nutrient deficiencies were assessed by measuring changes in foliar nitrogen and phosphorus concentrations following fertilization. We used three approaches to derive the nutritional status of trees: (1) critical nutrient concentrations, (2) nitrogen / phosphorus ratios, or (3) vector diagnostic analysis. We have shown that 18 years after scarification, nitrogen and phosphorus deficiencies have been reduced. At both sites, phosphorus was found to be more deficient than nitrogen. The effect of scarification was more beneficial in a maritime climate than in a continental climate.